Mild steel with an unprotected surface contains iron atoms that are chemically reactive and capable of undergoing electrochemical reactions with a variety of chemical species including water vapor and acidic gasses to form iron oxides. As a result of the differences in iron oxide crystal structures relative to those found in mild steel, iron oxides adhere poorly to the substrate and, as they are dislodged, expose still more steel to electrochemical oxidation. Efforts to protect steel structures from corrosion and, ultimately, structural failure or perforation, are a continuing and expensive burden.
A traditional method of preventing electrochemical oxidation of steel by acidic atmospheric gases and water vapor is simply coating the steel substrate with a barrier material that is relatively impervious to atmospheric oxidants. Such barrier coatings include, for example, various paints, aluminum, zinc, lead, tin and alloys thereof, aluminum oxide, ceramics and other materials. Passive barrier coatings are, however, compromised by any coating imperfections or subsequent damage that will allow oxidants to contact the underlying steel and initiate the corrosion process.
Another method for suppressing the corrosive electrochemical oxidation of steel involved providing a sacrificial coating that is more electrochemically reactive than the underlying steel substrate. Galvanic corrosion protection commonly uses metals such as zinc, aluminum and mixtures thereof to oxidize and thereby prevent underlying steel electrochemical oxidation. These sacrificial coatings, however, suffer from a degradation rate proportional to their oxidant exposure and because the thickness of the galvanic coating is necessarily limited, one must anticipate the sacrificial coating degradation rate for determining the projected useful service life of a steel article protected in such a manner. Weld points and coating imperfections further lessen the operational lifetime of a steel substrate provided with a sacrificial coating.
Additionally, the corrosion resistance of steel substrates can be improved by forming an intermetallic surface layer that is inherently corrosion resistant relative to steel. Although aluminum-steel alloys such as iron aluminide are well-known, controlling the reaction of a steel surface with a metal coating to form an intermetallic surface alloy of sufficient thickness and composition to provide corrosion resistance remains a challenge. Furthermore, the ductility, strength and weldability of such intermetallic alloys relative to an underlying steel substrate can create subsequent fabrication issues.
Steel used in automobile manufacture is typically formulated, treated and/or coated in order to suppress corrosion. A number of techniques and materials have been developed over the years for improving the corrosion resistance of the steel including, for example, hot dipping the steel in a bath of molten aluminum or aluminum alloy. Hot dip aluminum coated steel typically exhibits improved resistance to salt corrosion and has been used in, for example, automotive exhaust systems and body panels.
Increasing temperatures associated with the introduction of catalytic converters and other combustion management techniques have tended to increase both the temperature and the corrosiveness of the resulting combustion gases. The need for improved high temperature corrosion resistance led some manufacturers to begin replacing aluminum coated low carbon or low alloy steels with aluminum coated chromium alloy steels.
One of the mechanisms providing the improved corrosion resistance of the aluminum coated steels is the heat induced diffusion or transfer of at least a portion of the aluminum coating layer into the predominately iron base to form an intermetallic Fe—Al alloy layer. As will be appreciated by one skilled in the art, the particular structure, morphology and stoichiometry of the resulting Fe—Al alloy depends on a number of factors including, for example, the initial composition and the specific heat treatment procedure utilized in forming the alloy. As will also be appreciated by those skilled in the art, the structure of aluminum coated steels used in higher temperature applications may continue to change over the life of the part, a fact which will tend to guide the selection and formation of the initial structure and composition.
As noted above, one category of methods for manufacturing corrosion resistant coated steel involves subjecting the base steel to a “hot dip” in a bath of molten coating metal including, for example, aluminum, zinc and/or alloys and combinations thereof. One variation of the basic hot-dip technique, sometimes referred to as the Sendzimir process, prepares the steel by passing it through an oxidizing furnace where it is heated, without atmosphere control, to a temperature on the order of 1600° F. (870° C.). The heated steel is withdrawn from the furnace into air or another oxidizing ambient and allowed to form a surface oxide. The oxidized steel is then introduced passed through a reducing furnace containing a reducing atmosphere, for example, a mixture of hydrogen and nitrogen, and maintained under these conditions for a period sufficient to bring the strip to a reaction temperature of at least about 1350° F. (732° C.) and thereby reduce the surface oxide. The reduced steel surface is then typically maintained under a non-oxidizing until it is introduced into a selected molten metal bath.
Another variation of the basic hot-dip technique, sometimes referred to as the Turner or Selas process, involves passing the steel through a furnace heated to a temperature of at least 2200° F. (1204° C.) while maintaining a furnace atmosphere with little or no free oxygen and at least 3% excess combustibles. The steel is maintained under this environment for a period sufficient to heat the steel surface to a temperature of at least about 800° F. (427° C.) while suppressing surface oxidation. The heated steel is then moved into a reducing furnace under a hydrogen-nitrogen atmosphere and cooled to a temperature appropriate for introduction into the molten coating metal bath. Still others have proposed reducing or eliminating nitrogen from the surface treatment, particularly with chromium steels.
One of the difficulties associated with the hot dip coating method, particularly with respect to hot dip coating of aluminum and its alloys, is ensuring that the steel surface is sufficiently “wettable” by the molten coating metal so that a uniform coating layer may be formed during continuous manufacturing. One technique proposed for improving the wettability of steel containing one or more alloying metals, for example, no more than 5% chromium, no more than 3% aluminum, no more than 2% silicon and/or no more than 1% titanium, by heating the steel to a temperature above 1100° F. (593° C.) in an oxidizing atmosphere to form a surface oxide layer and then treating the oxide with reducing conditions in an effort to obtain a surface layer exhibiting an iron matrix containing a relatively uniform dispersion of oxides of the alloying element(s).
Despite various approaches, the limited wettability of molten aluminum coatings, particularly on stainless steels remains a concern in the industry. Poorly adhering hot dip aluminum coatings may exhibit flaking or crazing of the coating during metal deformation. Post dip heat treatments and rerolling techniques have been used to improve the initial adherence of the aluminum layer. Other manufacturers have adopted batch type hot dip coating processes, spray coating processes and/or extended dip duration, each of which can adversely impact throughput and/or uniformity.
Other approaches for improving the corrosion resistance have also included subjecting the aluminum coated steel to a chromate treatment and/or increasing the thickness of the aluminum coating itself. In higher temperature applications, e.g., those in which the resulting article will be exposed to temperatures of 300° C. or more, however, the chromate film produced provides less corrosion resistance and thicker aluminum coatings can exhibit peeling (delamination) as the steel is deformed during subsequent manufacturing processes.
One method for improving the adhesion and/or the corrosion resistance involves heat treating the aluminum coated steel under conditions that produce alloyed regions. The addition of a heat treatment process, however, increases the complexity of the manufacturing process and introduces additional variability in the quality and adhesion of the resulting aluminized steel. As will be appreciated by those skilled in the art, the process of diffusing aluminum into steel is not accurately reflected by simple diffusion models but rather by more complex reaction diffusion. The various localized compositions resulting from such a process may, in some instances, complicate later fabrication processes including, for example, stamping and/or welding.
Although generally referenced throughout this disclosure as an Fe—Al system, those skilled in the art will also appreciate that other alloying elements will typically be present in the dominate iron and aluminum compositions and will have greater or lesser impact on the diffusion system depending on their nature and concentration. A number of other elements and combinations thereof have been used in forming aluminum alloys and/or may be present as contaminates or introduced during processing. Antimony, in trace amounts (0.01 to 0.1 ppm), can be used instead of bismuth to counteract hot cracking in aluminum-magnesium alloys. Beryllium (typically up to 0.1%) can be used in aluminum/magnesium alloys for reducing oxidation at elevated temperatures, improving adhesion of the aluminum film to steel and for suppressing formation of deleterious iron-aluminum complexes. Low-melting-point metals such as bismuth, lead, tin, and cadmium may be added to improve machinability of aluminum alloys by promoting chip breaking and lubricating the cutting tool; boron may be used as a grain refiner and for improving conductivity by precipitating vanadium, titanium, chromium, and molybdenum, becoming more effective when used with an excess of titanium.
Cadmium may accelerate the rate of age hardening, increase strength, and increase corrosion resistance of aluminum-copper alloys and, at levels of 0.005 to 0.5%, has been used to reduce the time of aging of aluminum-zinc-magnesium alloys. Carbon may be present as an impurity and form oxycarbides and carbides with aluminum or other impurities and, as Al4C3 decomposes in the presence of water and water vapor, has been associated with surface pitting. Chromium may be present as a minor impurity in commercial grade aluminum and has a pronounced effect on electrical resistivity or, in amounts generally not exceeding 0.35%, may be used in aluminum-magnesium, aluminum-magnesium-silicon and aluminum-magnesium-zinc alloys to control grain structure, to prevent grain growth in aluminum-magnesium alloys, and to suppress recrystallization in aluminum-magnesium-silicon and aluminum-magnesium-zinc alloys during hot working or heat treatment.
Aluminum-copper alloys containing 2 to 10% Cu, typically with one or more other additions, form an important family of aluminum alloys that tend to respond to solution heat treatment and subsequent aging with increased in strength and hardness and a decrease in elongation. The main benefit of adding magnesium to aluminum-copper alloys is the increased strength possible following solution heat treatment and quenching. Cast aluminum-copper-magnesium alloys containing iron are typically characterized by dimensional stability and improved bearing characteristics, as well as by high strength and hardness at elevated temperatures. In certain compositions, however, iron concentrations as low as 0.5% may be associated with reduced tensile properties in the heat-treated condition, particularly if the silicon content is not sufficient to tie up the iron as FeSi constituents.
Hydrogen has a higher solubility in molten aluminum than in the solid at the same temperature. As a result, certain of the preconditioning methods above in which the steel surface is maintained under a reducing atmosphere, primary gas porosity can form during solidification and has been associated with secondary porosity, blistering, and high-temperature deterioration (advanced internal gas precipitation) during heat treating and may play a role in grain-boundary decohesion during stress-corrosion cracking. Hydrogen level in aluminum melts may be controlled by fluxing with hydrogen-free gases and/or by vacuum degassing. Small quantities (0.05 to 0.2%) of indium may have a significant influence on the age hardening of aluminum-copper alloys, particularly those with lower copper content, e.g., 2 to 3% Cu.
Iron is the most common impurity found in aluminum. It has a high solubility in molten aluminum and is therefore easily dissolved at all molten stages of production. The solubility of iron in the solid state is very low (˜0.04%) and therefore, most of the iron present in aluminum over this amount appears as an intermetallic second phase in combination with aluminum and often other elements. Lead is normally present only as a trace element in commercial grade aluminum, but may be added at levels of about 0.5%, typically in a 1:1 atomic ratio with bismuth to improve machinability.
The solid solubility of nickel in aluminum does not exceed 0.04% and in excess quantities will generally be present as an insoluble intermetallic, usually in combination with iron. Nickel levels of, for example, up to 2%, tend to increase the strength of high-purity aluminum but can also result in corresponding reductions in ductility. Nickel may be added to aluminum-copper or aluminum-silicon alloys for improving hardness and strength at elevated temperatures and reducing the coefficient of expansion of the resulting composition.
Silicon, after iron, is the highest impurity level in electrolytic commercial aluminum (0.01 to 0.15%) and, in wrought alloys, may be used with magnesium at levels up to 1.5% to produce Mg2Si in heat-treatable alloys. Magnesium added to aluminum-zinc alloys tends to improve the strength potential of the resulting alloy system, especially in the range of 3 to 7.5% Zn, with the magnesium and zinc forming MgZn2 and providing an increased response to heat treatment relative to the binary aluminum-zinc system. Adding copper to the aluminum-zinc-magnesium system, together with small amounts of chromium and manganese, produces some of the highest-strength aluminum-base alloys commercially available. It is believed that the copper increases the aging rate by increasing the degree of supersaturation and, possibly, through nucleation of the CuMgAl2 phase.